The objective of the proposed research is twofold, 1) to extend current knowledge on the nature and role in man of the leukocyte antigens and of the antibodies which these engender, and 2) to apply this information to the transplantation of human organs. In this program antisera which define new antigenic determinants will be sought in parous women, deliberately immunized individuals, and recipients of homografts. The role of specific leukocyte antigens and the influence of circulating antibodies in graft recipients will be evaluated. The quantitative distribution of specific antigens in various tissues will be determined and their ontogeny will be explored.